1. Field of the Invention
The present invention relates to a forming method and a forming apparatus. More specifically the present invention relates to a forming method and a forming apparatus in which a raw material powder comprising a fine powder or a granulated powder made therefrom is supplied into a cavity of a die, and the raw material powder supplied in the cavity is pressed to form a compact.
2. Description of the Related Art
In a press forming method, a die is formed with a cavity opening upward. A feeder box containing a raw material powder is placed above the cavity. The raw material powder is supplied by gravitational fall from the feeder box into the cavity. Then, the supplied raw material powder is pressed into a compact by an upper punch and a lower punch. An advantage of this method is that a compact of a high density can be obtained. According to the press forming method, in general, an amount of a binder to be used can be smaller than in an injection molding method or an extrusion molding method. Further, an amount of time required to perform a cycle of formation is also smaller. For these reasons, the press forming method is widely used.
When the press forming method is used to manufacture a small compact, the cavity of the die must have an area of an opening which is made accordingly and therefore considerably small, causing a difficulty that the raw material powder will not fall easily into the cavity. This is due to a phenomenon known as the bridging phenomenon, which is unique to a powder material. The bridging phenomenon makes unstable the amount of supply of the raw material powder into the cavity, making difficult to manufacture the compact satisfying a dimensional requirement. Further, the supply of the raw material powder into the cavity takes a longer time, increasing the amount of time required to perform the cycle of pressing operation.
In order to avoid the bridging phenomenon, there is employed a method of adding a binder to a powder thereby making a granulated powder having a greater grain diameter than the original powder grain (See Japanese Patent Laid-Open No. 8-20801, Japanese Patent Laid-Open No. 8-20802, and Japanese Patent Laid-Open No. 9-287001, for example). The granulated powder has a dramatically smaller contact area among granules, having a remarkably improved flowability. As a result, many small ceramic parts are now manufactured from the granulated powder, by using the press forming method.
On the other hand, a development is made also for a forming apparatus to avoid the bridging phenomenon, by utilizing a magnetic field or an ambient pressure difference, for example, in sucking the raw material powder into the cavity. Specifically, as a method of using the pressure difference, a proposal is made, in which the lower punch is quickly lowered when the feeder box comes above the cavity so as to create a partial vacuum within the cavity for sucking the raw material powder. In another proposal, the die is provided with a vent hole for sucking air from inside the cavity so that the raw material powder is supplied into the cavity under partial vacuum.
However, even if the granulated powder is used according to the former proposal, there is still a limit to catch up with further miniaturization of the parts, while there is a difficulty in further increasing a speed of the formation.
On the other hand, according to the latter proposal in which a relatively large pressure difference is created between inside and outside of the cavity for sucking the raw material powder, it is possible to quickly supply the raw material powder into the cavity. However, there is a narrow gap between the die and the lower punch from which a high pressure gas is discharged, allowing the raw material powder to build up in the gap. This can cause damage to the die when the lower punch is moved relative to the die, or cause seizure between the lower punch and the die. These problems interfere with continuous formation. Further, said sucking method can cause a fire accident, due to an excessive friction during the operation if the raw material powder is bound between the lower punch and the die and if the raw material powder is a rare-earth alloy powder.